Accurate load loss measurements of large power transformers at low power factors are difficult because of the presence of large quadrature component of current. The acceptable accuracy limits when measuring load loss in large power transformers are important to manufacturers and utilities since there is a penalty (as high as $5000/kilowatt) for every kilowatt of load loss exceeding the guaranteed value. For a power measurement that is accurate to 1 percent of 0.01 power factor, a technique with an overall accuracy of (0.01.times.1%) or 0.01 percent is required. This is equivalent to a total phase error, of not more than 100 rad anywhere in the measurement system, usually in the voltage and current transducers, or if there are no phase errors, an accuracy of 100 ppm (parts per million) of full scale in the wattmeter. Considering the stringent high accuracy requirement and the high penalty, it is important that loss measuring systems be calibrated after installation and recalibrated on a regular basis. Calibration of a test system and more recently not only that of a test system but also of its principal components have been described in a "A comparison of instrumentation for measuring the losses of large power transformers," Malewski et al., IEEE Trans. Power App. Syst., Vol. PAS-102, No. 6 pp 1570-1573, June 1983 and "Calibration of test systems for measuring power losses of transformers," Mehta et al., IEEE Trans. Power Delivery, Vol. PWRD-1, No. 4, pp. 215-22, Oct. 1986. System-based calibration is done by comparing the results of loss measurement with those of more accurate test system on the same transformer. This calibration method usually provides a system-based verification at only one voltage and one current range and at a particular power factor determined by the transformer under test. An alternative method would be the use of a standard load with a known stable loss, providing a reference power to the test system. Ideally, the standard load should be operable over different voltage, current, and power factor ranges. Such a standard load would provide a means for characterizing the accuracy of transformer loss measuring systems over different voltage, current, and power factor ranges.